The present invention relates to a method of manufacturing a magnetic recording medium having a non-magnetic substrate with an electrolessly deposited nickel-phosphorus (Nixe2x80x94P) coating thereon. The present invention has particular applicability to high areal density magnetic recording media exhibiting low noise and high coercivity.
Nickel (Ni) platings, particularly electroless Ni platings or deposits, enjoy technological applicability in various industries, such as the electronic, oil and gas, aerospace, machinery, automobile and magnetic recording media industries. Electroless Ni is employed in the metal finishing industry for various metal substrates, including steel, copper, aluminum and alloys thereof. Conventional electrolessly deposited Nixe2x80x94P platings exhibit desirable physical and chemical properties, such as hardness, lubricity, appearance, and corrosion resistance. An amorphous Nixe2x80x94P plating is conventionally applied to a non-magnetic substrate, such as aluminum (Al) or an Al-alloy substrate in manufacturing magnetic recording media.
In operation, a magnetic disk is normally driven by the contact start-stop (CSS) method, wherein the head begins to slide against the surface of the disk as the disk begins to rotate and, upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducing head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head begins to slide against the surface of the disk again and eventually stops in contact with and pressing against the disk. Thus, each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too smooth, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as xe2x80x9chead crash.xe2x80x9d Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as xe2x80x9ctexturing.xe2x80x9d Convention texturing involves mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers. The surface of an underlayer can also be textured, and the texture substantially replicated in subsequently deposited layers.
It is recognized, however, that electroless metal plating, such as electroless Nixe2x80x94P plating of a substrate, does not achieve a coating exhibiting a desired degree of surface smoothness, particularly the degree of smoothness necessary to satisfy the high areal recording density objectives of current magnetic recording media. In addition, the memory disk industry requires amorphous Nixe2x80x94P coatings that remain substantially non-magnetic, i.e., that do not crystallize. The transformation of electrolessly deposited amorphous Nixe2x80x94P coatings to the crystalline form during subsequent processing results in the formation of a thin magnetic Nixe2x80x94P layer with an attendant interruption in the magnetic field, thereby rendering the magnetic storage disks non-functional. In addition, crystalline boundaries associated with magnetic Nixe2x80x94P coatings provide high activity sites for chemical attack in a corrosive or moist environment.
The lower the intrinsic thermal stability of the Nixe2x80x94P coating, the lower the temperature at which it converts to the crystalline magnetic form during subsequent processing at elevated temperatures. The conversion of paramagnetic Nixe2x80x94P coatings to the magnetic form readily occurs in post plating processing, such as baking, texturing and sputter deposition. Moreover, during pre-sputter cleaning, the Nixe2x80x94P layer undergoes selective dissolution of the nickel, thereby leading to a P-enriched surface, e.g., an elevation in the P content from about 12 wt. % to about 20 wt. %. As a result, the surface layer becomes crystallized at a lower temperature than the bulk of the layer, resulting in the formation of a surface magnetic layer during subsequent processing, e.g., sputter deposition. The low intrinsic thermal stability exacerbated by nickel depletion during cleaning and/or localized heating not only leads to surface magnetization but also alters the smoothness of the substrate surface leading to head crash. During conventional processing, the detection of magnetic nickel occurs at temperatures as low as 310xc2x0 for 10 minutes at or near laser textured bumps.
There exists a need for methodology enabling the manufacture of magnetic recording media having a thermally stable Nixe2x80x94P layer. There exists a particular need for methodology enabling the manufacture of magnetic recording media comprising a non-magnetic substrate having a paramagnetic amorphous Nixe2x80x94P coating thereon which is thermally stable at sufficiently high temperatures to enable higher temperature sputter deposition for higher coercivity.
An advantage of the present invention is a method of electrolessly depositing an amorphous Nixe2x80x94P coating exhibiting high thermal stability and having an ultra-smooth as-deposited surface.
According to the present invention, the foregoing and other advantages are achieved by a method of manufacturing a magnetic recording medium, the method comprising: electrolessly depositing an amorphous nickel-phosphorous (Nixe2x80x94P) coating on a non-magnetic substrate employing a plating bath comprising aluminum (Al) and/or copper (Cu) ions in an effective amount to achieve an as-deposited average surface roughness (Ra) less than about 10 xc3x85 and a magnetic formation temperature not less than about 330xc2x0 C.; and cleaning the Nixe2x80x94P coating with an acidic agent having a pH less than about 4 or an alkaline agent having a pH greater than about 12 without any substantial depletion of nickel from the surface of the Nixe2x80x94P coating and without lowering the magnetic formation temperature.
Embodiments of the present invention include laser texturing a Nixe2x80x94P coated substrate at a temperature no less than about 330xc2x0 C. without any substantial magnetic transformation of the amorphous Nixe2x80x94P coating. Embodiments of the present invention further include cleaning the laser textured substrate with an acidic agent having a pH less than about 4 or an alkaline agent having a pH greater than about 12.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.